X-ray or γ-ray radiation, optical radiation, ultrasound waves and magnetic field have been used to examine and image biological tissue. X-rays or γ-rays propagate in the tissue on straight, ballistic lines, that is, their scattering is negligible. Thus, imaging is based on evaluation of the absorption levels of different tissue types. For example, in roentgenography the X-ray film contains darker and lighter spots. In more complicated systems, such as computerized tomography (CT), a cross-sectional picture of human organs is created by transmitting X-ray radiation through a section of the human body at different angles and by electronically detecting the variation in X-ray transmission. The detected intensity information is digitally stored in a computer that reconstructs the X-ray absorption of the tissue at a multiplicity of points located in one cross-sectional plane.
Near infra-red radiation (NIR) has been used to study biological tissue non-invasively, including oxygen metabolism in the brain, finger, or ear lobe, for example. The use of visible, NIR and infra-red (IR) radiation for medical imaging may have several advantages: In the NIR or IR range the contrast factor between a tumor and a tissue is much larger than in the X-ray range. In addition, the visible to IR radiation is preferred over the X-ray radiation since it is non-ionizing and thus, potentially causes fewer side effects. However, the visible or IR radiation is strongly scattered and absorbed in biological tissue, and the migration path cannot be approximated by a straight line, making inapplicable certain aspects of cross-sectional imaging techniques.
Optical spectroscopy has been used to monitor and image tissue blood oxygenation and volume by measuring absorption of oxyhemoglobin and deoxyhemoglobin in the near infrared (NIR) wavelength region. Below 700 nm, light is strongly absorbed by hemoglobin, and above 900 nm, it is strongly absorbed by water. By making differential measurements at either side of the isosbestic point of oxyhemoglobin and deoxyhemoglobin absorbance (near 800 nm), it is possible to quantify the blood oxygenation and volume levels. Typically, these measurements are made at 750 nm and 830 nm.
NIR spectrometry adapted to the principles of computerized tomography has been used for in vivo imaging. This technique utilizes NIR radiation in an analogous way to the use of X-ray radiation in an X-ray CT. The X-ray source is replaced by several laser diodes (or other light sources) emitting light in the NIR range. The NIR-CT uses a set of photodetectors that detect the light that had migrated in the imaged tissue. The detected data are manipulated by a computer in a manner similar to the detected X-ray data in an X-ray CT. Different NIR-CT systems have recognized the scattering aspect of the non-ionizing radiation and have modified the X-ray CT algorithms accordingly.
Brain tissue has been particularly studied by many burgeoning technologies, wherein MRI is truly versatile as being capable of imaging hemodynamic and metabolic signals in a unique fashion. PET has similar possibilities of large chemical specificity governed by the combination of lifetimes and radiation from radioactive isotopes. Other methods give highly specialized signals, for example, MEG and EEG that have respectively high and low resolution for neurophysiological signals. Optical tomography is somewhat more quantitative with respect to hemodynamic changes and has latent possibilities for measuring neuronal signals.
Furthermore, the propagation of near infrared light through biological tissue such as the brain and breast has been experimentally studied and theoretically modeled. Accurate theoretical models are based on Monte Carlo representations of the diffusion equation and on analytic expressions that show propagation into the gray matter of the brain in adults and especially in neonates. This propagation of light into cranial tissue has been verified by clinical measurements of the presence of X-ray CT identified cranial hematomas at depths of about 3-4 cm. Detection of the oxygenation state and amount of hemoglobin has been the goal of tissue oximetry and quantitative results are obtained by time and frequency domain devices. However, single volume determination of optical parameters of a highly heterogeneous system such as the human brain may give only a fraction of the signal of a localized focal activation already shown to be highly localized by fMRI (functional Magnetic Resonance Imaging).
The optical systems are relatively simple, safe, portable and affordable as required by today's health care industry. There are several optical examination and imaging devices that have been used for imaging functional activity of adult, full-term and pre-term neonate brain. These optical examination and imaging systems are described in U.S. Pat. Nos. 5,353,799; 5,853,370; 5,807,263, 5,820,558, which are incorporated by reference. These optical systems do not require subject immobilization (as do MRI and PET), nor do they require multi-subject averaging of data. The images are acquired in less than half a minute and show two dimensional resolution of blood changes to better than a centimeter. In these optical systems, however, light sources and light detectors are mounted directly next to the examined tissue or the light is coupled to the tissue using light guides (e.g., optical fibers). In these optical systems, however, the subject has to wear the optical coupler or probe. Furthermore, the optical probe has to provide electrical insulation to prevent electrical shock to the subject.
There is still a need for optical examination and imaging systems for examining various types of biological tissue including the brain or breast tissue.